What can lead to habitat alteration in aquatic environments?

The construction of barriers in aquatic environments, such as dams and levees, can significantly alter habitats. These barriers disrupt the natural flow of water, affecting sediment transport, nutrient distribution, and the migration patterns of aquatic organisms. For example, fish that rely on free-flowing rivers to spawn may be unable to reach their breeding grounds due to dams. This alteration can lead to changes in biodiversity, population dynamics, and the overall health of the aquatic ecosystem.

While improved agricultural practices can lead to some positive outcomes, such as reduced soil erosion and better water management, they can also contribute to habitat alteration indirectly through runoff and nutrient loading into waterways. Similarly, natural cleaning of lakes, often through processes like sedimentation and biological activity, typically restores environmental health rather than alter it. Seasonal changes, though impactful, occur within the natural cycles of ecosystems and do not constitute alteration in the same sense that man-made barriers do. Thus, the most direct impact on habitat alteration in the context provided is indeed through the construction of barriers.